Tell me what is wrong with my set up.

[LowBuckX]

Stock booster and 68 Corvette disc/disc Master cylinder with 1 inch bore.
Front Calipers have 2.8 bore single piston
Rear calipers have 2.38 bore single piston <--- correction 2 13/32 or 2.406

Am I having a fluid Volume issue?

[TonyL]

what kind of lines do you have?

what are the symptoms, what's it do?

[LowBuckX]

Lines are as follows
Front 3/16 from Master to a "T" then 3/16 to calipers.
Rear. 1/4 from Master to adjustable prop then to axle then 3/16 out to the calipers

Problem is I have bled over a gallon through the rears and have a soft pedal still. Someone metioned in another post that I may have a volume issue.

[TonyL]

oh I see. My first thought was, older rubber lines are swelling when the pedal is depressed. I've seen that happen a few times, Switching to braded hoses and hardline help alot. Im no expert on volumes, but im sure someone else may chime in soon.

[Hidro]

The corvettes used 4 piston calipers in 1968.Using a single piston the volume wont be the same.

-Ed

[LowBuckX]

The corvettes used 4 piston calipers in 1968.Using a single piston the volume wont be the same.

-Ed

Can you elaborate? Too much .. not enough..

[Hidro]

4 piston calipers use less volume and less pressure than a single piston caliper. (from wilwood website)

-Ed

[LowBuckX]

Then what Master should I be running?

[LowBuckX]

[6'9"Witha69]

A larger MC may help. I just re read this and the other thread and noticed something. You never indicated if you removed th hold off valve in the rear lines. You didn't mention it in the order, just wondering if it is still there. second thing may be residual pressure valves. I HAD to run one in my rear line to keep the pedal up. It would bleed off FAST without it. If it sat for more than a day I had almost no pedal. An indicator is if you can pump a couple times and get a good pedal, then as you hold it firm, it goes soft under your foot. It can feel like a bad MC sometimes.

[LowBuckX]

No hold off.
I was hoping for a confirmation on the residual checks. I will add one this weekend.
If that doesnt give me a pedal I will add a larger master I guess.

[OHCbird]

What calipers are you running?

You shouldn't need RPVs in a properly bled system; any residual pressure on the rear caliper will drag the pads. Do you have an adjustable prop valve plumbed in?

[sinned]

Let’s start from the beginning as there is a lot of misinformation going around here. You state you have a "soft" pedal, what exactly does that mean? Is that engine off or engine on, are you running a booster or manual system? How is the pedal height? If this condition exists while driving, does it improve with a few strokes and what pad compound are you running? Do you have a residual valve at all (yes, even disc systems need residual valves albeit rated quite a bit lower than the drum versions)?

Brake line size will have no affect or pedal sponginess. It can cause a touchy pedal and affect pedal height to some degree but never a “soft” pedal. As Tony alluded to already, how are the flexible brake hoses?

[LowBuckX]

All flexi lines are new.
I have an adjustable prop plimbed in.
No metering block and no check valve.
Soft pedal is with engine off. With engine on I can get a pedal but with a few pumps.
front calipers are stock 72 nova units
rear are GM Metric "fronts" from an 85 Camaro (front)

just now I got back from the garage. I took calipers off the brackets suspended them in the air with bleeder on top. gravity then vacum bled them while taping and shaking them.(speed bleeders removed and standard in place. . 0 air from either side.

Please explain "PEDAL HEIGHT"

Sleight pressure starts from about 1/4 of total stroke. Or 1.5 inch from top

[sinned]

Pedal height is how far from the top the pedal sits when fully depressed. I’m still confused about "soft" pedal. Is it spongy feeling, or does sink once it initially grabs? What type of booster if any and you state the pedal gets better after a few pumps with the engine running, does it not get better with the engine off?

[LowBuckX]

Pedal height is 1.5 inch from the bottom. after 2 pumps it is at 2.5. Wait about 10 seconds then push iit will go back to 1.5.

This is also from pedal to carpet not to the metal.
Soft pedal meaning. My pedal used to be a rock with the engine off. Now with the discs and the Vette master it has nothing.

[sinned]

I am thinking an air problem, you state you have bleed the brakes quite a bit, fronts too? What bleeding method are you using?

[LowBuckX]

Ive pumped them out with just the speed bleeders and a container. Ive used a Mightyvac.
I dont have any help to do a 2 man bleed. I dont see any air when useing the mightyvac on the fronts either.

I am building a pressure bleeder that looks like the motive bleeder. Would that help?

[LowBuckX]

Well I did a butt load of research and the best answer was given in 2004 by David pozzi on a camaro board.

A 2 lb residual pressure valve wouldn't hurt at all, might help a little.

A couple of problems arise using the S-10 calipers.

1. they have a much larger bore than is needed for a rear caliper, this uses up fluid volume pumped by the master cyl and you have excess pedal travel problems. You also have to use an adjustable proportioning valve to lower the rear pressure.

2. Those calipers are "Low Drag" type with a different seal angle that retracts the pistons more than usual. It is used with a special "Quick Take Up" (QTU) master cyl with a third, much larger piston at the rear of it that displaces more fluid volume until 100 psi line pressure is reached, then the third piston pressure above 100 psi is returned to the reservoir and the smaller MC pistons will develop the pressure. think of it as a two stage master cyl, the first portion of travel delivers high volume at low pressure, moves the caliper pistons out to touch the rotors, then the two pressure pistons take over.

Your choices are to either use smaller rear caliper bores, or think about the QTU master cyl. This master cyl has a much larger rear portion where it pilots into the brake booster so you would have to find an 80's Camaro booster to use along with it. It won't fit your firstgen booster.

I would rather see you find a smaller bore caliper. It would be great if there was one with the same mounts and rotor thickness as what you have now, but I don't know of any.

Maybe you can find something. Otherwise I remember my 89-91 IROC had aluminum rear calipers and used the S-10 type front calipers, so the F/R ratios should be correct for your application. Rear discs were an option, also pre-89 Camaros and Firebirds had cast iron rear calipers that some of them had self adjuster problems but those calipers are probably more common. The Cadillac Seville had the same iron calipers. The rotors are probably around .8" thick, but I'm guessing. The axle flange OD may need to be turned down to fit the rotor.

Before you give up on what you have, make sure your brakes have been well bled and the calipers are not twisting when you apply the brakes sitting still. If they are twisting that is using up pedal travel too. Make sure all bleed screws are pointing up so all the air is bled out.
David

But all is not lost Ive also found what maybe the solution. 1977 monza booster with a 1986 disc disc master.
If it dont work Im done for the year and I will be ebay ing off my my newly aquirred shiney junk.

[andrewb70]

The pistons in your rear calipers are HUGE! Especially when you compare them to the fronts. My theory is that your rear brakes are barely working. Your soft pedal is a result of not enough fluid being pumped to the rear calipers with a single stroke. This is confirmed by the fact that pumping the pedal helps. If you get a bigger MC sush as a 1 1/8" bore that will solve your soft pedal issue, however it will create a host another problem. Since your rear caliper bore is so large your rear brakes will now be prone to premature lock up. Your piston areas are so out of whack that a prop valve may not even be able to have the required range of adjustment to tune out the premature braking.
This is also common with guys running the Explorer rear brakes on Ford 9" rear ends. Thats just my take on the whole thing....

Andrew